Apparatus and method for transmitting data with conditional zero padding

ABSTRACT

An apparatus and method for transmitting data with conditional zero padding is provided. In accordance with an embodiment of the disclosure, a transmitter transmits data to a receiver by transmitting symbols such that each symbol is preceded by a cyclic prefix of a fixed length and the symbol conditionally includes enough zero padding to avoid ISI (Inter-Symbol Interference) between consecutive symbols. In some implementations, if the fixed length for cyclic prefixes is long enough to avoid ISI between consecutive symbols, then the symbols may omit zero padding. Otherwise, the symbols may include enough zero padding to avoid ISI between consecutive symbols. The zero padding may be zero tail or zero head.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/015,641, filed on Feb. 4, 2016, entitled “Apparatus and Method forTransmitting Data With Conditional Zero Padding”, which is acontinuation of U.S. Pat. No. 9,313,063 filed Jan. 30, 2015, entitled“Apparatus and Method for Transmitting Data With Conditional ZeroPadding”, the entire disclosures of which are hereby incorporated byreference.

FIELD OF THE DISCLOSURE

This application relates to wireless communication, and moreparticularly to transmitting data with conditional zero padding.

BACKGROUND

When a transmitter transmits a signal to a receiver, the transmissionmight travel over more than one path to the receiver due to diffractionand/or reflection off of objects in the surrounding environment. Thesignal travelling over the longest path will arrive at the receiverafter the signal travelling over the shortest path. This delay is knownas channel delay spread. If the channel delay spread is not accountedfor, then there may be ISI (Inter-Symbol Interference) betweenconsecutive symbols, which can make it difficult or even impossible torecover the data at the receiver.

SUMMARY OF THE DISCLOSURE

An apparatus and method for transmitting data with conditional zeropadding is provided. In accordance with an embodiment of the disclosure,a transmitter transmits data to a receiver by transmitting symbols suchthat each symbol is preceded by a cyclic prefix of a fixed length andthe symbol conditionally includes enough zero padding to avoid ISIbetween consecutive symbols. The zero padding may be zero tail or zerohead.

In some implementations, if the fixed length for cyclic prefixes is longenough to avoid ISI between consecutive symbols, then the symbols mayomit zero padding. Otherwise, the symbols may include enough zeropadding to avoid ISI between consecutive symbols. In specificimplementations, the symbols include an amount of zero tail so that alength of the zero tail in addition to the fixed length of the cyclicprefixes is greater than or equal to a channel delay spread fortransmitting data to the receiver.

According to another embodiment of the disclosure, if a transmitterdetermines that a cyclic prefix length associated with symbols in atransmission is insufficiently long to prevent ISI between adjacentsymbols, then the transmitter inserts zero padding into at least one ofthe symbols to mitigate the ISI. The zero padding may involve insertingzero tail into fixed length symbols.

Other aspects and features of the present disclosure will becomeapparent, to those ordinarily skilled in the art, upon review of thefollowing description of the various embodiments of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described with reference to the attacheddrawings in which:

FIG. 1 is a schematic of a wireless system featuring a wireless deviceand a wireless network, in accordance with an embodiment of thedisclosure;

FIG. 2 is a schematic representing formats of data that may betransmitted by the wireless device of FIG. 1;

FIGS. 3A and 3B are schematics representing alternative formats of datafor comparison;

FIG. 4 is a flowchart of a method for transmitting data based on achannel delay spread, in accordance with an embodiment of thedisclosure;

FIG. 5 is a flowchart of a method for transmitting data in a manner thatadapts to changes in the channel delay spread, in accordance with anembodiment of the disclosure;

FIG. 6 is a schematic of a wireless system featuring multiplesimultaneous transmissions, in accordance with an embodiment of thedisclosure;

FIG. 7 is a schematic representing multiple simultaneous transmissions,in accordance with an embodiment of the disclosure;

FIG. 8 is a schematic representing other formats of data that may betransmitted by the wireless device of FIG. 1;

FIG. 9 is a schematic showing example components that may be implementedin a transmit function, in accordance with an embodiment of thedisclosure;

FIG. 10 is a graph showing transmission power of a transmitter;

FIG. 11 is a flowchart illustrating a method of mitigating ISI; and

FIG. 12 is a schematic representing other formats of data for mitigatingISI.

DETAILED DESCRIPTION OF EMBODIMENTS

It should be understood at the outset that although illustrativeimplementations of one or more embodiments of the present disclosure areprovided below, the disclosed systems and/or methods may be implementedusing any number of techniques, whether currently known or in existence.The disclosure should in no way be limited to the illustrativeimplementations, drawings, and techniques illustrated below, includingthe exemplary designs and implementations illustrated and describedherein, but may be modified within the scope of the appended claimsalong with their full scope of equivalents.

Introduction

Referring first to FIG. 1, shown is a schematic of a wireless system 30featuring a wireless device 10 and a wireless network 20. The wirelessdevice 10 has a wireless access radio 11, circuitry implementing atransmit function 12, a processor 13, a computer-readable medium 14, andmay have other components that are not specifically shown. The wirelesssystem 30 may have many other wireless devices in addition to thewireless device 10, but they are not specially shown. The wirelessnetwork 20 has wireless components that are not specifically shown. Notethat the wireless network 20 may include wired components even thoughthe wireless network 20 is described as being wireless. The wirelessnetwork 20 is deployed in an environment in which there may be someobstacles, for example a building 25 or other obstacles not specificallyshown.

Operation of the wireless system 30 will now be described by way ofexample. The wireless device 10 is capable of communicating with thewireless network 20 using the wireless access radio 11. Suchcommunication may for example involve email, text messaging, telephony,web browsing, etc. Since the communication is wireless and there may bevarious obstacles such as the building 25, transmission from thewireless device 10 may include more than one transmission path 21, 22.In the illustrated example, transmission from the wireless device 10 tothe wireless network 20 includes a direct path 21 and an indirect path22 caused by reflection off the building 25. Since the indirect path 22is longer than the direct path 21, the transmission following theindirect path 22 arrives at the wireless network 20 later than thetransmission following the direct path 21. The delay between thetransmissions over the transmission paths 21, 22 is known as channeldelay spread. If the channel delay spread is not properly accounted for,then there may be ISI (Inter-Symbol Interference) between consecutivesymbols that may make it difficult or even impossible to recover thedata.

In accordance with an embodiment of the disclosure, the wireless device10 has circuitry implementing the transmit function 12 with the wirelessaccess radio 11 for transmitting data in a manner that reduces thelikelihood of ISI. In particular, the transmit function 12 operates totransmit symbols such that each symbol is preceded by a cyclic prefix ofa fixed length and the symbol conditionally includes enough zero paddingto avoid ISI between consecutive symbols. The zero padding may be zerotail or zero head as described in further detail below. In someimplementations, the amount of zero tail or zero head is communicated tothe receiver to facilitate recovery of the transmitted data at thereceiver.

There are many possibilities for implementing the wireless device 10.The wireless device 10 might be a mobile terminal such as a tablet,smartphone, vehicle phone, etc. Alternatively, the wireless device 10might be a fixed terminal and/or form part of a machine or a homeappliance such as a refrigerator. Note that the wireless system 30 mayhave a mix of mobile terminals and fixed terminals.

There are many possibilities for the circuitry implementing the transmitfunction 12 of the wireless device 10. In some implementations, thecircuitry includes the processor 13, which is configured to implementthe transmit function 12 when instructions recorded on thecomputer-readable medium 14 are executed by the processor 13. In otherimplementations, the circuitry includes a DSP (Digital SignalProcessor), an FPGA (Field Programmable Gate Array), an ASIC(Application Specific Integrated Circuit) and/or a microcontroller. Moregenerally, the circuitry implementing the transmit function 12 includesany appropriate combination of hardware, software and firmware.

There are many possibilities for the wireless access radio 11 of thewireless device 10. In some implementations, the wireless access radio11 includes a receiver and a transmitter. The receiver may be coupled toa receive antenna while the transmitter is coupled to a transmitantenna. In some implementations, the wireless access radio 11 includesmore than one receiver and more than one transmitter. The transmitter(s)and receiver(s) may be coupled to a processing unit, for example a DSP,for processing signalling.

Although the illustrated example described above with reference to FIG.1 focuses on transmission from the wireless device 10, it is to beunderstood that embodiments of the disclosure are also applicable totransmission from a network node (e.g. base station) of the wirelessnetwork 20. In such alternative implementations, the network node wouldpossess a transmit function similar to the transmit function 12 of thewireless device 10. More generally, embodiments of the disclosure areapplicable to any appropriate transmitter, regardless of whether thetransmitter is a wireless device or a network node of a wirelessnetwork.

Referring now to FIG. 2, shown is a schematic representing formats ofdata that may be transmitted by the transmit function 12 of the wirelessdevice 10. As shown at 2-1, the wireless device 10 may transmit symbolssuch that each symbol is preceded by a cyclic prefix of a fixed length.For each symbol, the cyclic prefix is a copy of the end of the symboland is positioned before the symbol. The fixed length is a time value(i.e. measured in seconds) and is equal to a number of samples takenfrom the end of the symbol divided by a sampling rate. In situations inwhich the channel delay spread is less than the fixed length of thecyclic prefixes, then the wireless device 10 may rely on the cyclicprefixes as shown at 2-1 to avoid ISI between consecutive symbols. Thisis because the cyclic prefixes are long enough to avoid adjacent symbolsfrom overlapping due to the channel delay spread. However, if thechannel delay spread is greater than the fixed length of the cyclicprefixes, then the wireless device 10 pads the symbols with zeros asshown at 2-2. The zero padding can be zero tail (i.e. zeros padded atthe end of each symbol) as shown, or alternatively zero head (i.e. zerospadded at the beginning of each symbol). The zero padding is notdesigned to replace the cyclic prefixes, but to compensate for cyclicprefixes that are insufficient for avoiding ISI. The zero padding formspart of the symbols, which means that symbols may contain less data whenthere is zero padding.

In some implementations, the amount of zero padding is chosen so that alength of the zero padding in addition to the fixed length of the cyclicprefixes is greater than or equal to the channel delay spread. Thisenables ISI to be avoided. Note that the length of zero padding for atransmitted symbol is a time value (i.e. measured in seconds) and mayhave discrete values based on how many zeros are present, as the numberof zeros are whole numbers as explained in further detail below withreference to FIG. 9. In specific implementations, the zero padding ischosen as a minimum amount of zero tail to avoid ISI between consecutivesymbols. Introducing further zero padding is possible, but this isunnecessary and results in greater overhead and less data per symbol.Note that when the length of zero tail is less than the fixed length ofcyclic prefixes, the cyclic prefixes will contain non-zero samples thatcan be used for assisting synchronization. However, when the length ofzero tail is greater than the fixed length of cyclic prefixes, thecyclic prefixes will contain zeros from the zero tail.

Embodiments of the disclosure refer to conditionally including zero tailor zero head to “avoid ISI” between consecutive symbols. It is to beunderstood that avoiding ISI generally means that all problematic ISI isevaded. However, it is possible that some benign ISI, which may not evenbe detectable, may remain. Problematic ISI means ISI that has ameaningful effect on recovering the data. Problematic ISI can make itdifficult or even impossible to recover the data. Benign ISI generallymeans ISI that has no meaningful effect on recovering the data. Nospecial considerations are required for dealing with benign ISI. ISI maybe avoided between a first symbol and a subsequent symbol when mostenergy from the first symbol, due to channel delay spread, falls withinthe cyclic prefix of the subsequent symbol, rather than reaching thebody of the subsequent symbol. It should also be noted that embodimentsof the disclosure can be used to mitigate but not eliminate problematicISI. For example, some embodiments may be implemented so that only apercentage of the problematic ISI is eliminated.

The fixed length of the cyclic prefixes is relatively small so thatthere is a relatively small overhead, for example 7%, but of courseother fixed lengths are possible. The fixed length is a time value thatmay be predetermined and does not change over time. Furthermore, thesymbols also have fixed length, which allows the format of data shown at2-1 and the format of data shown at 2-2 to share the same alignment.Sharing the same alignment can be advantageous from a system designperspective, for example because neighbouring base stations may besynchronized and use the same system parameters such as sub-carrierspacing. This may greatly reduce complexity of the wireless system 30.Note that this alignment would not be possible if the cyclic prefixeswere allowed to have variable length while the symbols maintain constantlength as shown in FIG. 3A. Omitting cyclic prefixes altogether andrelying on zero padding as shown in FIG. 3B could allow alignment.However, omitting the cyclic prefixes is not desirable because thecyclic prefixes are useful for time-domain synchronisation. Also, if theoverhead granularity for zero padding is relatively large, for example8.3% in the case of 12 subcarriers, then total overhead may be largerthan needed.

Methods for Transmitting Data

Referring now to FIG. 4, shown is a flowchart of a method fortransmitting data based on a channel delay spread. This method may beimplemented by a transmitter, for example by the transmit function 12 ofthe wireless device 10 shown in FIG. 1. Alternatively, this method maybe implemented by a network node (e.g. base station) of a wirelessnetwork. More generally, this method may be implemented by anyappropriate transmitter.

The method starts under the presumption that the transmitter is totransmit data to a receiver and the transmitter is aware of the channeldelay spread for transmitting the data to the receiver. At step 4-1,based on the channel delay spread for transmitting data to the receiver,the transmitter determines whether a fixed length for cyclic prefixes islong enough to avoid ISI between consecutive symbols. For example, thetransmitter may determine that the fixed length for cyclic prefixes issufficient for avoiding ISI between consecutive symbols only if thefixed length for cyclic prefixes is equal to or longer than the channeldelay spread. Since the fixed length for cyclic prefixes and the channeldelay spread are both time values measured in seconds, they may becompared to one another.

If the fixed length for cyclic prefixes is long enough to avoid ISIbetween consecutive symbols, then at step 4-2 the transmitter transmitssymbols such that each symbol is preceded by a cyclic prefix of thefixed length. See for example the format of data shown at 2-1 in FIG. 2.Note that zero padding does not need to be used because it introducesoverhead and is unnecessary for avoiding ISI between consecutivesymbols.

However, if the fixed length for cyclic prefixes is not long enough toavoid ISI between consecutive symbols, then at step 4-3 the transmittertransmits symbols such that each symbol is preceded by a cyclic prefixof the fixed length and the symbol has enough zero tail or zero head toavoid ISI between consecutive symbols. See for example the format ofdata shown at 2-2 in FIG. 2. Note that zero padding is applied becausethe cyclic prefixes are not sufficient for avoiding ISI betweenconsecutive symbols. In specific implementations, there is a minimumamount of zero tail so that a length of the zero tail in addition to thefixed length for cyclic prefixes is greater than or equal to the channeldelay spread. Additional zero padding is possible, but it wouldintroduce additional overhead and is unnecessary for avoiding ISIbetween consecutive symbols.

There are many ways in which the transmitter may be aware of the channeldelay spread for transmitting the data to the receiver. In someimplementations, the transmitter is pre-configured with the channeldelay spread. Such pre-configuration may stem from previous measurementstaken during deployment of the transmitter or the receiver. In otherimplementations, the transmitter measures the channel delay spread of adata reception that previously occurred, and due to the reciprocalnature of the directions in a bi-directional communications channel, thetransmitter can assume that the channel delay spread for transmitteddata is equal to the channel delay spread for received data. Note thatthe channel delay spread may change for example if the transmitter orthe receiver moves, or if there are changes in the surroundingenvironment. In some implementations, the transmitter determines thechannel delay spread for transmitting data to the receiver on an ongoingbasis in order to adapt to changes in the channel delay spread. Anexample of this is described below with reference to FIG. 5.

Referring now to FIG. 5, shown is a flowchart of a method fortransmitting data in a manner that adapts to changes in the channeldelay spread. This method may be implemented by a transmitter, forexample by the transmit function 12 of the wireless device 10 shown inFIG. 1. Alternatively, this method may be implemented by a network node(e.g. base station) of a wireless network. More generally, this methodmay be implemented by any appropriate transmitter.

At step 5-1, the transmitter determines a channel delay spread fortransmitting data to a receiver. In some implementations, this isaccomplished based on pre-configurations as explained above, based onmeasurement of receive signals as explained above, or a combinationthereof. The transmitter then transmits data in a manner that accountsfor the channel delay spread at steps 5-2 through 5-4. These steps aresimilar to steps 4-1 through 4-3 described above with reference to FIG.4 and so their description is not repeated here.

As noted above, the channel delay spread may change for example if thetransmitter or the receiver moves, or if there are changes in thesurrounding environment. Therefore, in some implementations, if at step5-5 there is more data to transmit, then the method loops back to step5-1 so that the channel delay spread can be determined on an ongoingbasis. Thus, the transmitter can adapt transmission of data at steps 5-3and 5-4 according to changes in the channel delay spread. For example,the transmitter can apply a minimum amount of zero tail for eachiteration of the method to avoid ISI between consecutive symbols. Theminimum amount of zero tail can for example include no zero tail (i.e.transmission at step 5-3) when the channel delay spread is small enoughto permit this case, or some zero tail (i.e. transmission at step 5-4)when the channel delay spread is larger than the fixed length for cyclicprefixes. In some implementations, when changing the amount of zero tailor zero head, the change is communicated to the receiver in advance ofthe change to facilitate the receiver to recover the data.

Multiple Simultaneous Transmissions

Although the examples described above focus on a transmitter with onetransmission at a time, it is to be understood that embodiments of thedisclosure are applicable to multiple simultaneous transmissions.Multiple simultaneous transmissions from a transmitter are possibleusing different frequency sub-bands as described below with reference toFIGS. 6 and 7.

Referring now to FIG. 6, shown is a schematic of a wireless system 60featuring multiple simultaneous transmissions. The wireless system 60has a first base station 63, a second base station 64, and may haveother base stations and other network components that are notspecifically shown. The wireless system 60 also has a first wirelessdevice 61, a second wireless device 62, and may have other wirelessdevices that are not specifically shown.

Operation of the wireless system 60 will now be described by way ofexample. The first base station 63 transmits data to the first wirelessdevice 61 as shown at 6-1, and transmits data to the second wirelessdevice 62 as shown at 6-2. As shown in FIG. 7, these multipletransmissions occur at the same time, but at different frequencies.

In this example, the first data transmission as shown at 6-1 has cyclicprefixes and symbols without any zero tail or zero head because thefixed length of the cyclic prefixes is longer than the channel delayspread for transmitting data to the first wireless device 61. Incontrast, the second data transmission as shown at 6-2 has both cyclicprefixes and zero tail because the fixed length of the cyclic prefixesis shorter than the channel delay spread for transmitting data to thesecond wireless device 62. Note that the channel delay spread fortransmitting data to the second wireless device 62, in this particularexample, is greater than the channel delay spread for transmitting datato the first wireless device 61 due to the different locations of thewireless devices 61, 62, and the data transmission as shown at 6-2accounts for this by using zero tail. Notwithstanding this, bothtransmissions have the same symbol length. Also, both transmissions maybe aligned for greater interference cancellation. In particular, sinceboth transmissions have the same symbol length, they may be synchronizedin time, and they have the same sub-carrier spacing.

Note that some receivers may support CoMP (Coordinated Multipoint) andreceive the same transmission from more than one transmitter. Forexample, as shown in the illustrated example, the second wireless device62 receives the second transmission as shown at 6-2 and a thirdtransmission from a second base station 64 as shown at 6-3. These twotransmissions share the same data and the same zero tail as shown inFIG. 7 at 6-2. The amount for zero tail for these two transmissions maybe designed for a composite channel delay spread so that ISI may beavoided for both transmissions. This may involve some coordination amongthe base stations 63, 64 transmitting data to the second wireless device62 to determine a composite channel delay spread. The composite channeldelay spread depends on various factors such as the respective distancesbetween the base stations 63, 64 and the second wireless device 62.

Some receivers may not support CoMP and may receive transmissions fromonly one transmitter at a time. For example, as shown in the illustratedexample, the first wireless device 61 receives only the firsttransmission from the first base station 63 as shown at 6-1. In thisscenario, there may be no need for coordination among the base stations63, 64.

Wireless Technologies

Embodiments of the disclosure are applicable to many possible wirelesstechnologies. One possible wireless technology is DFT-s-OFDM (DiscreteFourier Transform spread Orthogonal Frequency-Division Multiplexing),which is also known as SC-FDMA (Single Carrier Frequency-DivisionMultiple Access). For DFT-s-OFDM implementations, each symbol that istransmitted is an OFDM symbol. Single carrier based technologies such asIEEE (Institute of Electrical and Electronics Engineers) 802.11ad arealso possible. For IEEE 802.11ad implementations, each symbol that istransmitted is a sequence of modulated data, and each cyclic prefix is apilot symbol as described in further detail below. Other wirelesstechnologies are possible and are within the scope of this disclosure.More generally, embodiments of the disclosure are applicable to anysuitable single carrier based technology.

It is to be understood that embodiments of the disclosure are applicableto wireless technologies that utilize cyclic prefixes but do not referto them as being cyclic prefixes. An example of this is IEEE 802.11ad,which is described to utilize “pilot symbols” as cyclic prefixes. Inorder to illustrate this point, reference is made to FIG. 8, which is aschematic representing other formats of data that may be transmitted bythe wireless device 10 of FIG. 1 for the case of IEEE 802.11ad. Thefirst format of data shown at 8-1 of FIG. 8 corresponds to the firstformat of data shown at 2-1 of FIG. 2, except that the first format ofdata shown at 8-1 of FIG. 8 uses “pilot symbols” as cyclic prefixes.Likewise, the second format of data shown at 8-2 of FIG. 8 correspondsto the second format of data shown at 2-2 of FIG. 2, except that thesecond format of data shown at 8-2 of FIG. 8 uses “pilot symbols” ascyclic prefixes. Each “pilot symbol” for a data symbol is cyclic andprefixes the data symbol. Thus, for the purpose of this disclosure, theterm “cyclic prefix” is considered to cover “pilot symbol” or any othercyclic prefix that may be referred to using other terminology.

As noted above for FIG. 1, the circuitry implementing the transmitfunction 12 of the wireless device 10 may include any appropriatecombination of hardware, software and firmware. However, the manner inwhich the transmit function 12 is implemented may depend on the wirelesstechnology that is utilized. Example components of a transmit functionare described below with reference to FIG. 9 for a case of DFT-s OFDM.

Referring now to FIG. 9, shown is a schematic showing example components90 that may be implemented in a transmit function in the case ofDFT-s-OFDM. It is to be understood that components 90 may form part of atransmit function of a wireless device or a network node (e.g. basestation) of a wireless network. The components 90 include a paddingcomponent 91, a DFT (Discrete Fourier Transform) component 92, asubcarrier mapping component 93, an IFFT (Inverse Fast FourierTransform) component 94, a parallel to serial component 95, and mayinclude other components that are not specifically shown. It is to beunderstood that the components 90 are very specific for exemplarypurposes only.

Operation of the components 90 will now be described by way of example.The padding component 91 conditionally pads a stream of data symbolswith a zero tail or zero head length that is determined in accordancewith an estimated channel delay spread to mitigate ISI betweenconsecutive OFDM symbols to be transmitted. In the case of zero padding,the stream of data symbols includes one or more zeros, which are timeslots with no data symbol. Note that the total OFDM duration remainsfixed regardless of the zero padding performed. Before the data that hasbeen conditionally padded is transmitted, the data is mapped in thefrequency domain to a frequency sub-band. To this end, the OFT component92 transforms the data from a time domain into a frequency domain sothat the subcarrier mapping component 93 can map the data in thefrequency domain to the frequency sub-band. The IFFT component 94 thentransforms the data back from the frequency domain to the time domainprior to the parallel to serial component 95 performing conversion intoa serial signal for transmission over a communication channel. Note thatthe cyclic prefix is fed into the serial signal by the parallel toserial component 95 along with the data and zero padding.

The serial signal includes OFDM symbols, where each OFDM symbol ispreceded by a cyclic prefix and the OFOM symbol has enough zero paddingto avoid 151 between consecutive OFOM symbols. As previously noted forFIG. 2, the length of zero padding for a transmitted symbol is a timevalue (i.e. measured in seconds). In this case, the length of zeropadding for the OFDM symbols have discrete time values based on how manyzeros are present. In particular, for each OFDM symbol, the paddingcomponent 91 determines how many zeros there will be in the stream ofdata symbols used to generate the OFDM symbol. Since the number of zeros(i.e. number of time slots with no data symbol) is a whole number, theOFOM symbols, which are derived from streams of data symbols, havediscrete time values based on the number of zeros.

Note that the zero padding, whether zero tail or zero head, whentransmitted provides for low transmission power, but such lowtransmission power may not be zero. To illustrate this point, referenceis made to FIG. 10, which is a graph 100 showing transmission power of atransmitter for DFT-s-OFDM. The graph 100 shows that transmission powerfor zero tail and zero head is significantly reduced, but not actuallyzero. However, for single carrier systems such as IEEE 802.11ad, truezeros may be used. In any event, the terms “zero padding,” “zero tail”and “zero head” as used throughout this disclosure are not to beinterpreted to necessarily require zero transmission power.

Another Method for Mitigating ISI

Referring now to FIG. 11, shown is another method 200 for mitigatingISI. This method may be implemented by a transmitter, for example by atransmit function of a wireless device. Alternatively, this method maybe implemented by a network node (e.g. base station) of a wirelessnetwork. More generally, this method may be implemented by anyappropriate transmitter.

In step 202, a determination is made that the length of the cyclicprefix of a symbol is insufficiently long to prevent ISI betweenadjacent symbols. In step 204, zero padding is inserted into the symbolitself to mitigate the effects of ISI. In some embodiments, step 202 caninclude using a channel delay spread value in the determination. Thechannel delay spread can be estimated in step 206, and the determinationof step 202 can be done in accordance with the estimated channel delayspread in step 208. To provide for the zero padding in step 204, in step210 zeros can be added to the end of the symbol (i.e. zero tail). Oneskilled in the art will appreciate that zero headers can be used inplace or, or in addition to the zero tail.

In some embodiments, the zero padding is provided for each symbol of atransmission. An example of this has been described above with referenceto FIG. 2. However, it is to be understood that embodiments in whichonly a subset of the symbols have zero padding are possible. An exampleof this will be described below with reference to FIG. 12.

Referring now to FIG. 12, shown is a schematic representing otherformats of data for mitigating ISI. As shown at 12-1, a transmitter maytransmit symbols such that each symbol is preceded by a cyclic prefix assimilarly described above with reference to FIG. 2. However, if thechannel delay spread is greater than the length of the cyclic prefixes,then the transmitter may pad the symbols with zeros as shown at 12-2. Inthis example, the zero padding includes both zero tail and zero head forevery other symbol (e.g. symbols 2, 4, 6, 8 . . . ) while remainingsymbols have no zero padding at all. Note that lSl can be mitigatedusing the cyclic prefixes and zero padding even though half of thesymbols have no zero padding at all.

The format of data shown at 12-2 is irregular in the sense that datapayload differs from symbol to symbol. In particular, the symbols havingthe zero padding have smaller payload than the symbols having no zeropadding. Note that other irregular formats of data that combine cyclicprefixes and zero padding are possible. For example, with respect to theformat of data shown at 12-2, any one of the symbols having both zerotail and zero head could alternatively have only zero head provided thatthe subsequent symbol has zero head instead of no zero padding. Variouscombinations of zero tail and zero head are possible such that ISI canbe mitigated. In general, zero padding may be inserted into at leastsome of the symbols to mitigate the lSl.

As a further example, with reference back to FIG. 2, although zero tailis used for each symbol at 2-2, in alternative implementations, zerotail is omitted from the last symbol of the transmission. This isbecause the zero tail of the last symbol of the transmission is notactually needed to mitigate ISI.

In some embodiments, the symbols have a fixed length, and the cyclicprefixes also have a fixed length. Examples of this have been describedabove. However, other embodiments are possible in which irregular symbollengths and/or irregular cyclic prefix lengths are possible.

Computer Readable Medium

In accordance with another embodiment of the disclosure, there isprovided a non-transitory computer readable medium having computerexecutable instructions stored thereon for execution on a processor of atransmitter so as to implement any of the methods described herein. Thenon-transitory computer readable medium might for example be an opticaldisk such as a CD (Compact Disc), a DVD (Digital Video Disc), or a BD(Blu-Ray Disc). Alternatively, the non-transitory computer readablemedium might for example be a memory stick, a memory card, a disk drive,a solid state drive, etc. Other non-transitory computer readable mediaare possible and are within the scope of this disclosure. Moregenerally, the non-transitory computer readable medium can be anytangible medium in which the computer executable instructions can bestored.

Numerous modifications and variations of the present disclosure arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the disclosuremay be practised otherwise than as specifically described herein.

We claim:
 1. A method comprising: transmitting a signal comprising aplurality of fixed length symbols each having a respective cyclic prefixhaving a fixed cyclic prefix length; wherein when the fixed cyclicprefix length is insufficiently long to prevent ISI between adjacentsymbols, the fixed length symbol including an amount of zero paddingnecessary to avoid ISI, wherein a time duration of the zero padding inaddition to the fixed cyclic prefix length is substantially equal to andnot greater than a channel delay spread.
 2. The method of claim 1further comprising: Estimating a channel delay spread; inserting thezero padding based on the estimated channel delay spread.
 3. The methodof claim 1 wherein the step of inserting zero padding includes insertinga zero tail into a fixed length symbol.
 4. The method of claim 3 whereinthe step of inserting zero padding further includes inserting a zerohead into the fixed length symbol.
 5. The transmitter of claim 1 furthercomprising: a channel delay spread estimator for estimating a channeldelay spread; wherein the zero padding inserter inserts the zero paddingbased on the estimated channel delay spread.
 6. The transmitter of claim1 wherein the zero padding inserter inserts a zero tail into a fixedlength symbol.
 7. The transmitter of claim 6 wherein zero paddinginserter further inserts a zero head into the fixed length symbol.
 8. Anapparatus comprising: a transmitter configured to transmit a signalcomprising a plurality of fixed length symbols each having a respectivecyclic prefix having a fixed cyclic prefix length; a zero paddinginserter configured to, when the fixed cyclic prefix length isinsufficiently long to prevent ISI between adjacent symbols, insert ineach fixed length symbol an amount of zero padding necessary to avoidISI, wherein a time duration of the zero padding in addition to thefixed cyclic prefix length is substantially equal to and not greaterthan a channel delay spread.